Hot gas path components of gas turbines and aviation engines, particularly turbine blades, vanes, nozzles, seals and stationary shrouds, operate at elevated temperatures, often in excess of 2,000° F. The superalloy compositions used to form hot gas path components are often single-crystal compositions incorporating significant amounts of tantalum (Ta).
The present invention is an improvement to the class of alloys disclosed and claimed in U.S. Pat. No. 6,416,596 B1, issued Jul. 9, 2002 to John H. Wood et al.; which was an improvement to the class of alloys disclosed and claimed in U.S. Pat. No. 3,615,376, issued Oct. 26, 1971 to Earl W. Ross. Both patents are assigned to the assignee hereof and are incorporated by reference in their entirety. One known superalloy composition within the above class of alloys is referred to herein as “GTD-111.” GTD-111 has a nominal composition, in weight percent of the alloy, of 14% chromium, 9.5% cobalt, 3.8% tungsten, 1.5% molybdenum, 4.9% titanium, 3.0% aluminum, 0.1% carbon, 0.01% boron, 2.8% tantalum, and the balance nickel and incidental impurities. GTD-111 is a registered trademark of General Electric Company.
GTD-111 contains substantial concentrations of titanium (Ti) and tantalum (Ta). In certain conditions, Eta phase may form on the mold surfaces and in the interior of the casting, which, in some cases results in the formation of cracks. An attribute of the alloys disclosed and claimed in U.S. Pat. No. 6,416,596, including GTD-111, is the presence of “Eta” phase, a hexagonal close-packed form of the intermetallic Ni3Ti, as well as segregated titanium metal in the solidified alloy. During alloy solidification, titanium has a strong tendency to be rejected from the liquid side of the solid/liquid interface, resulting in the segregation (local enrichment) of titanium in the solidification front and promoting the formation of Eta in the last solidified liquid. The segregation of titanium also reduces the solidus temperature, increasing the fraction of gamma/gamma prime (γ/γ′) eutectic phases and resulting micro-shrinkages in the solidified alloy. The Eta phase, in particular, may cause certain articles cast from those alloys to be rejected during the initial casting process, as well as post-casting, machining and repair processes. In addition, the presence of Eta phase may result in degradation of the alloy's mechanical properties during service exposure.
In addition to the formation of Eta, the class of alloys claimed in U.S. Pat. No. 6,416,596 is susceptible to the formation of detrimental topologically close-packed (TCP) phases (e.g., μ and σ phases). TCP phases form after exposure at temperatures above about 1500° F. TCP phases are not only brittle, but their formation reduces solution strengthening potential of the alloy by removing solute elements from the desired alloy phases and concentrating them in the brittle phases so that intended strength and life goals are not met. The formation of TCP phases beyond small nominal amounts results from the composition and thermal history of the alloy.
Articles and methods having improvements in the process and/or the properties of the components formed would be desirable in the art.